" it would be too big for IMG tags"
OK, that explains it. I missed that in the training manual. Thanks for the tip adding L or s to the URL. I won't remember it past next Tues, unless Imgur has a highly visible "shortcuts / tips" sheet.

I clicked "add image" on the forum's toolbar (I'm using Fx 84 - Linux), then pasted the external link between [img]&[/img] - copied from Imgur's URL, after the image was uploaded.
It just displayed the word "Image."
So I just pasted the image URL by itself & it displayed.

Are you actually linking to the image itself or the page the image is on? when using the IMG tags.

[url]http://forums.mozillazine.org/faq.php?mode=bbcode#f5r0[/url]

Your image is https://i.imgur.com/XsFSjHJ.png however it would be too big for IMG tags anyways with it being 1053 × 2065 pixels as it would cause scrolling.

Though with imgur you can make the image be a large or small thumbnail link to full size 1053 × 2065 image. Just add [b]l[/b] for large or [b]s[/b] for small at end of file name. https://i.imgur.com/XsFSjHJl.png or https://i.imgur.com/XsFSjHJs.png

Just quote the post to see how below is done for large thumbnail link to full size. The links should be on imgur when you uploaded.

[url=https://i.imgur.com/XsFSjHJ.png][img]https://i.imgur.com/XsFSjHJl.png[/img][/url]

In what application are you trying to do that?

Hovering the Img button shows, Insert Image [img]http://img_url[/img].
But inserting the URL from Imgur: [img]https://imgur.com/a/RqtNXZK[/img], [b]nothing[/b] but the word "Image" shows up in submitted post.
Tried it w/o the secure conn. - http; no diff.

Had to just enter the url w/o any codes around it, then it shows OK when click the live link.